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An interdisciplinary approach was used to investigate the facies and paleogeography of the Lower Devonian sedimentary sequence of the Alken quarry, Mosel Valley, Germany. This 87-m-thick sequence consists of stratified sandstones and sandy shales of the Nellenköpfchen Formation (uppermost Lower Emsian). Previous interpretations of the depositional environment include terrestrial, deltaic, and shallow-marine settings. Two distinct fossiliferous units contain abundant terrestrial plant remains and a diverse mixed terrestrial to marine fauna. Physical sedimentary structures are common throughout, whereas bioturbation is restricted mostly to the fossiliferous intervals. Erosional surfaces frequently separate the beds. Aside from ripple cross-stratification and parallel bedding, longitudinal inclined stratification is most common. Channel-fill structures are less frequent. Scour-and-fill structures exhibit marked disconformities of irregular shape on a smaller scale (dm). Mud-pebble lags at the base of laterally prograding cross-bedded layers, scour-and-fill structures, and drainage rills characterize the upper part of the section. Desiccation cracks, wind-induced striation, and water-level marks occur more sporadically in the exposure. The sedimentary structures and the paleontological information indicate a marine to brackish depositional environment that frequently was emergent. The presence of conspicuous channel-related structures reflects intertidal conditions along the coastal region of a presumed Hunsrück Island/Archipelago. Lagoons and estuaries were bordered by extended tidal flats, in which migrating channels frequently occurred. Terrestrial plant remains, however, indicate a position at the land/sea interface, which was characterized by a complex configuration of different environments. The accumulation of concentrated plant material may have been related to distinct meteorological events such as hurricanes.
Macrofauna abundance has been quantified across the Breistroffer interval (OAE 1d, Latest Albian) in the Vocontian basin (southeast France). This interval is characterized by a high abundance of ammonoids showing significant morphologic disparity. Twenty ammonoid genera are recognized and assigned to seven morphostructural groups. The aims of this study are to: (1) interpret abundance variations of ammonoid taxa in terms of taphonomic processes, sedimentary dilution, and paleoenvironmental factors; and (2) discuss the habitat and mode of life of some Late Albian ammonoids with respect to their morphology and abundance variations.
Most variations in ammonoid absolute abundance are not the result of post-mortem shell transport. High abundance within the Breistroffer interval is due partly to a low accumulation rate, which is related to a maximum-flooding stage. The different trends observed among ammonoid taxa can be interpreted partly in terms of response to changes in trophic conditions inferred from the study of calcareous nannofossils. Heteromorphs, which are dominant during mesotrophic conditions, could have been more competitive than involute/evolute planispirals (normal coiling) when paleoenvironmental conditions become more unstable. Lechites (orthocone), the most abundant taxon, probably inhabited epipelagic, distal paleoenvironments. This genus, interpreted as a vertical migrant in the water column, would have been able to move up when trophic conditions were good in surface waters, and down in order to avoid oligotrophic surface waters and to exploit food-rich layers in deeper waters. Turrilitoides and Mariella (torticones) may have been quasiplanktic, and inhabited mainly neritic paleoenvironments. These forms could have occupied more distal paleoenvironments when mesotrophic conditions prevailed in the surface water column. Anisoceras and Hamites (quasiplanktic U-shaped heteromorphs) mainly lived in distal, epipelagic habitats, and could have been more competitive compared with the torticones when oligotrophic conditions prevailed in surface waters. Among involute/ evolute planispiral ammonoids, Mortoniceras probably had a deep-nektonic mode of life and inhabited the lower part of the epipelagic zone. A stylized panorama of some Late Albian ammonoid habitats is proposed.
Paleontological data frequently are extracted from genetically and stratigraphically complex shell beds. It is therefore important to recognize taphonomic biases that can lead to major errors in paleoecological interpretations (e.g., on ancient local biodiversity). The strong influence of transport-related shell-size sorting on diversity estimates from single samples was studied in a transect of the middle Miocene Grund Formation (Lower Austria), which contains allochthonous, psammitic event beds with channel structures, sharp erosional bases, and graded bedding. These event beds are interpreted as proximal tempestites, and contain densely packed, polytaxic molluscan assemblages. The faunal composition and taphonomic features of shells indicate that transport occurred from wave- or current-agitated nearshore habitats into a pelitic, inner-shelf environment. The different skeletal concentrations contain a highly diverse molluscan fauna with 130 species identified from more than 4200 individuals. Although the quantitatively most-important species are the same in standardized samples from five different shell beds, species richness differs significantly among the three samples from the base of the transect and the two samples from its top. Diversity depends on size-sorting and therefore reflects the transport history of the individual tempestites, not the species richness of the original paleocommunity. Poorly sorted samples (indicating relatively minor transport) approximate the diversity of single samples of that environment better than well-sorted samples (which indicate stronger transport). Diversities of shelly assemblages from parautochthonous and allochthonous assemblages cannot be compared directly. Even comparisons among tempestites are problematic because transport intensity governs diversity. The intensity of any taphonomic process, however, is difficult to predict without detailed investigations. The use of samples from taphonomically complex shell beds for diversity comparisons can bias results, especially on the fine-scale level of local diversity patterns. Studies at such fine scales of resolution should consider the taphonomic framework of assemblages, which is necessary to recognize the dominant taphonomic factors and their intensities.
Three cores from the scleractinian coral Montastraea faveolata and one core from the scleractinian coral Siderastrea siderea from the Belize barrier and atoll reef complex, Central America, were analyzed with regard to sclerochronology (skeletal extension rates) and stable isotope geochemistry (δ18O and δ13C). The core material covers the time span from 1815 to 2000. The four coral time series were compared with available instrumental climate data, such as sea surface temperatures (GISST), cloud cover (GHCN), and precipitation (COADS). Skeletal extension rates measured in the cores average 8.5–14 mm/year in M. faveolata and 8 mm/year in S. siderea. No systematic correlations between extension rates and instrumental climate data were detected. Annual variation in oxygen isotopes is 0.6– 0.8‰, which accounts for monthly averages of sea-surface water temperature fluctuations of 3–4°C. In three cores from the Belize shelf and barrier reef, negative correlations of δ18O with the GISST 2.3b data were observed. Time series analyses of the oxygen isotope data in these three cores revealed a decadal periodicity (10–15 years), which are attributed here to the influence of the Atlantic sea surface temperature (SST) dipole variation. Three- to six-year periodicities, indicative of the El Niño Southern Oscillation (ENSO), in the same data are only weakly developed. Carbon isotopes of shallow-water corals also exhibit negative correlations with the GISST data. It is speculated that warmer years were characterized by increased cloud cover leading to reduced photosynthesis rates in the corals. Indeed, there is a negative correlation between δ13C and historic cloud-cover data. Carbon isotopes in the Belize shelf and barrier reef cores further exhibit general trends towards lighter values in time indicating the uptake of fossil fuel CO2 in the coral skeletons. A proxy data time series from a core from the restricted Turneffe Atoll lagoon differs from those of the Belize shelf and barrier reef cores, and there are fewer systematic correlations with historical climate and proxy data. This is probably a consequence of the strong influence of local environmental factors, which obliterate broader scale environmental parameters. First analyses of oxygen isotopes in fossil (Holocene) coral cores of M. faveolata indicate higher SSTs around 7,000 ybp in Belize as compared to older, younger, and modern samples.
Modern taphofacies analyses have been worked out for different mixed carbonate and siliciclastic sedimentary environments using molluscan faunas. Studies of how the combined effect of the physical, chemical, and biological characteristics of the environment of deposition results in a particular taphonomic signature on fossilizable remains must be done on Recent assemblages to understand taphonomic signatures of ancient faunas. Studies have defined modern taphofacies by erecting broad damage categories to which each sample is assigned. Others graphically characterize environments based on individual taphonomic indices (e.g., fragmentation, abrasion), rely on presence-absence data, or construct ternary taphograms for each taphonomic characteristic.
This study defines taphofacies based on a statistical treatment of the entire taphonomic signature of molluscs from several carbonate reef and lagoon systems in the northeastern Caribbean. The method uses non-metric multidimensional scaling ordination to test whether the combined taphonomic signature is recognizable across closely associated shallow carbonate environments. Results show that a statistical technique combining many taphonomic factors is a reliable method for deciphering taphonomic signature. It is most important, however, to test whether taphofacies defined using modern shells can be applied to the fossil record. Taphonomic traits from mollusc shells obtained from reef cores were added to the same statistical routine and assigned environments of deposition based on their taphonomic signature. Molluscs from within the cores suggest that the shelf and patch-reef environments on the SW coast of Puerto Rico have remained fairly stable up to at least 7,000 ybp. Shells were recovered that had distinct reef and open-shelf taphonomic signatures. Shells from cores from Buck Island, U.S. Virgin Islands had signatures that suggested migration of the reef over seagrass areas of the shelf, and/or landward into the lagoon behind Buck Island reef. Thus, not only does the total taphonomic signature mirror the environment of deposition, but the taphonomic signature is likely to be preserved well enough to be useful in making paleoenvironmental interpretations of fossil material.
The Middle Devonian (Emsian/Eifelian) Trout Valley Formation is exposed in the northeastern corner of Baxter State Park, Maine, and is noted for its abundant plant-fossil assemblages. However, to date no invertebrate macrofaunal assemblages have been reported in this fluvial-to-marine sequence; only isolated eurypterid parts have been reported. A previously undescribed outcrop of coarse- to medium-grained siltstone characterized by megaripples preserves a restricted, transported invertebrate assemblage. The macrofossils are randomly oriented and concentrated in the ripple crests. Similar sedimentological features in other parts of the stratigraphic section indicate an estuarine, tidally influenced depositional regime. The fossil assemblage is dominated by Phthonia sectifrons—an uncommon Devonian bivalve known previously from open-marine deposits. Few other fossil taxa occur. Typically associated with deep-shelf brachiopods, the presence of P. sectifrons in these estuarine deposits indicates that this taxon occupied a wider range of habitats, extending from the shallow silty shelf facies to more transitional settings. The facies associations in the Trout Valley Formation suggest that Devonian near-shore communities resembled modern bivalve-dominated near-shore communities.
The Early Miocene rise of the grass-dominated ecosystem is a plausible trigger for a sharp Miocene increase in accumulation of nonmarine diatomaceous sediment as well as diversification of nonmarine diatoms. This grassland radiation introduced a biogeochemical mechanism for enhancing widespread and sustained mobilization of usable silica and other nutrients. Volcanism was probably responsible for episodic nonmarine diatomaceous sediments from the advent of the oldest known nonmarine diatoms in the Late Cretaceous through the Oligocene. Although prolific Miocene volcanism was undoubtedly still important in the development of many diatomites, feedback from grassland colonization of volcanic soils may explain why diatomaceous sedimentation surged in the Miocene following a more sparse pre-Miocene record.
The initial rise of the grass-dominated ecosystem, increased nonmarine diatomite accumulation, and Early Miocene evolutionary radiations of nonmarine diatom taxa are at least approximately coeval. Although the earliest known grass is Paleocene, multiple lines of evidence, including mollic-epipedon paleosols, fossil occurrences of hypsodontic ungulate grazers, and fossil phytoliths, suggest that the grass-dominated ecosystem did not expand significantly until Early Miocene. The grassland radiation apparently was delayed until Middle Miocene in parts of Eurasia, Africa, and Australia. If that delay is real, the diatom/diatomite record in those regions should coincide with it. The onset of increased Miocene diatomite accumulation is as yet imprecisely dated, but coincidence with the rise of the grass-dominated ecosystem is predicted herein. Early Miocene diversifications of Actinocyclus and Thalassiosira diatoms are consistent temporally with grassland expansion where it is Early Miocene.
Subsequent adjustments in the silica cycle also may be attributed to grasslands. Nonmarine diatom radiations in the Late Miocene and Pliocene coincide with sharp regressions that may have released nutrients and soluble phytolith opal stored in Miocene soils and paleosols as well as dissolved silica in soil pore waters. Regressional erosive pulses of phytoliths provide a new explanation for low Ge/ Si ratios in marine diatoms during Pleistocene glacial intervals. Nonmarine diatoms from regressive intervals should record lower Ge/Si ratios than before and after those regressions because of phytolith contributions with low Ge/Si ratios. Late Miocene radiations of C4 and moist tall-grass ecosystems may have mobilized even more silica than the short, dry-climate Early Miocene grasses. Abundance of diatomite may have fluctuated in concert with changes in degree of volcanism, even after grassland expansion, but at substantially higher levels than before this new terrestrial ecosystem arose.